Determining gas concentration

ABSTRACT

Method and device for determining the concentration of one or more gases, e.g. O 2  or CO 2  or an anaesthetic gas, in a fluid e.g. a body fluid or a gas mixture. A membrane (12) permeable to the gases retains a solvent (22) e.g. dimethylsulphoxide. In contact with the solvent is a working electrode (24) of a surface area preferably less than 10 μm 2 . The potential of the working electrode is sept, over a range to reduce each of the one or more gases, and at a rate to minimize cross-reactions between gases and their reduction products.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and device for determining theconcentrations of one or more gases in a fluid. The fluid may be in theliquid or gas phase and is, for example, a body fluid such as serum. Theinvention is of primary interest for determining the concentrations ofoxygen and carbon dioxide, as well as nitrous oxide and certainanaesthetic gases. But the principles of the invention are applicable toother gases.

2. Discussion of Prior Art

The continuous measurement of O₂ and CO₂ in clinical medicine has led toa whole industry of measurement devices. In the blood, oxygen ismeasured by the amperometric Clark PO₂ electrode; and CO₂ is measured bythe potentiometric (glass electrode) Stow-Severinghaus electrode. Thustwo sensors, working on entirely different principles, have to beemployed whenever PO₂ and PCO₂ are measured. Blood-gas analyzerstherefore use two separate sensors. Intravascular measurements can onlybe made for PO₂, using Clark cells fabricated on the tip of a polymercatheter. It has proved impossible, so far, to miniaturize the glasselectrode, and so measure intravascular PCO₂, with the Stow-Severinghaustechnique. Paediatric intravascular oxygen sensors have beensuccessfully developed, first by D. G. Searle and then by Hoffman laRoche, for paediatric use, and these sensors are now manufactured byBiomedical Sensors Ltd. (High Wycombe).

In the gaseous phase, oxygen is measured with Clark-type sensors forsteady-state analysis (e.g. for anaesthetic machines); and by fastparamagnetic analyzers for breath-by-breath analysis. Expired CO₂ isalmost inevitably measured with an infrared analyzer.

Outside medicine, as the control of CO₂ increases in importance invarious technologies, there is an ever growing need for inexpensive CO₂sensors with high sensitivity and selectivity. Such examples include thefermentation industry in general; brewing; on-line industrialmonitoring; pollution measurement; CO₂ level measurement in largeauditoria; vehicle exhaust analysis, etc. In many instances it would bea great advantage to be able to measure O₂ simultaneously, with the samesensor measuring both O₂ and CO₂, and using the same analysis principle.

At potentials of the order of -0.5 to -1.0 V, against Ag/AgCl referenceelectrode, in an aprotic solvent such as DMSO, oxygen in solution isreduced by the reaction:

    O.sub.2 +e→O..sub.2.sup.-                           ( 1)

This superoxide radical is stable for short periods in non-aqueoussolvents. But it reacts rapidly with carbon dioxide, by a series ofreactions which may be summarised as:

    2O..sub.2.sup.- +2CO.sub.2 →C.sub.2 O.sub.6.sup.2-  ( 2)

At potentials in the range -1.5 to -2.5 V, dissolved carbon dioxide isreduced, initially by virtue of the reaction:

    CO.sub.2 +e→CO.sub.2..sup.-                         ( 3)

European patent 162622 describes a gas sensor and method which usedreactions (1) and (2) to provide a simultaneous determination of oxygenand carbon dioxide concentrations. There was described a pulsed CO₂titration technique, whereby the electrode surface was kept deliberatelylarge in order to produce enough O₂.⁻ to consume all the CO₂ present. Apulsed voltage sufficiently negative to reduce the O₂ molecule (but notsufficiently negative to reduce CO₂) was first applied to the electrodesurface, followed by an oxidizing pulse to oxidize those O₂.⁻ ions whichhad remained after reacting with the CO₂ molecules.

The problems with this technique were that a large cathode surface wasneeded, leading to high sample consumption; a complicated mathematicalrelationship was required to extract the CO₂ concentration, and themeasured O₂ concentration was complicated by the enhancement of itssignal from the chemical reactions (1) and (2) shown above.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a device and method bywhich these problems may be overcome. This is achieved by usingreactions (1) and (3) above under conditions to minimize interferencefrom reaction (2).

Apart from oxygen and carbon dioxide, there is also a need for improvedtechniques for determining the concentrations of other gases such asnitrous oxide (N₂ O) and gases used in anaesthesia. Devices for theclinical measurement of N₂ O and the anaesthetic gas halothane, based onamperometric electrochemical sensing, are known. For example, N₂ O andhalothane may be measured on silver cathodes in aqueous electrolytes(Clin. Phys. Physiol. Meas., 8 (1987) 3-38). However, isoflurane, thepopular anaesthetic gas, has heretofore been considered to beelectrochemically inert over a wide range of electrode materials in bothaqueous and non-aqueous solution (J. Electroanal. Chem. 244(1988)203-219) and thus unsuited to such techniques.

In particular, there is a need for equipment that is capable ofaccurately measuring the concentrations of a range of gases, eitherseparately or as components in a mixture of gases, and which is easy touse and relatively inexpensive. The present invention seeks to providesuch apparatus and a method for its use.

In one aspect the present invention provides a device for determiningthe concentrations of one or more gases in a fluid, comprising amembrane permeable to the gas or gases, a solvent for the gas or gasesand which is retained by the membrane, a working electrode having asurface area less than 10000 square microns in contact with the solvent,a counter and/or reference electrode in contact with the solvent, meansfor applying to the working electrode a potential and for sweeping thepotential over a range effective to reduce the gas or gases in thesolvent and at a rate sufficient to minimize the interfering effect ofany reaction between one gas and a reduction product of any other, andmeans for measuring the current generated at a predetermined potentialas indication of the concentration of the first gas, and where theconcentrations of two or more gases are being determined, means formeasuring the current generated at each of one or more furtherpredetermined potentials as an indication of the respectiveconcentrations of the one or more other gases present.

In another aspect the invention provides a method of determining theconcentrations of one or more gases in a fluid, which method comprisesapplying the fluid to one side of a membrane permeable to the gas orgases, the other side of the membrane retaining a solvent for the gas orgases,

using a working electrode having a surface area less than 10000 squaremicrons in contact with the solvent to apply a potential which is sweptover a range effective to reduce the gas or gases in the solvent,

wherein the rate of sweep of potential is sufficient to minimize theinterfering effect of any reaction between one gas and a reductionproduct of any other,

measuring the current generated at a predetermined potential as anindication of the concentration of the first gas, and, where theconcentrations of two or more gases are being determined, measuring thecurrent generated at each of one or more further predeterminedpotentials as an indication of the respective concentrations of the oneor more other gases present.

The device and method may be used to determine the concentration of asingle gas or of two, three or more gases simultaneously. In addition togases such as O₂, CO₂ and N₂ O, it has been found that theconcentrations of anaesthetic gases such as the following can also bedetermined by this means: ##STR1##

This finding is considered to be particularly surprising since, as notedabove, isoflurane has previously been regarded as electrochemicallyinert. However, we have now shown that such gases can be reduced using aworking electrode of the aforementioned size and a suitable solvent.

The fluid may be a gas or a liquid, e.g. a body fluid such as serum. Themembrane is selected according to the nature of the gas or gases underinvestigation. For O₂, CO₂ and/or N₂ O gases, for example, the membranemay suitably be of a material such as PTFE. In the case of volatileanaesthetic gases, however, the membrane would preferably be of an inertporous material (such as sintered glass) which would not be attacked bythe solvent. The solvent may be dimethylsulphoxide (DMSO), althoughother non aqueous solvents including acetonitrile and propylenecarbonate are possible. The solvent may contain a small amount of ascavenger, for example for the superoxide ion, such as water. Though notpreferred, it is possible and may be convenient to use a solventcontaining a minor proportion of water, e.g. up to 10% v/v or evenhigher. A conductivity improver such as TEAP may also be present. Theworking electrode may be of silver or carbon or platinum or morepreferably of gold and the counter electrode may be of platinum. Areference electrode, e.g. of Ag/AgCl, may be included in the system.

The potential of the working electrode is swept over a range which iseffective to reduce each of the gases in the solvent. The range to becovered is therefore dependent upon which gas or gases are involved inany given instance, but would typically be from -0.5 V to -2.5 V orgreater (i.e. more negative). For oxygen and carbon dioxide, forexample, the range is (as noted above) approximately -0.5 V to -2.5 V,though the reduction potentials of the two gases do depend on variousother factors. This sweeping is performed under conditions whichminimize the interfering effect of any reaction between one gas (carbondioxide) and a reduction product of the other (superoxide ion). In otherwords, the system is operated under conditions to minimize reaction (2)above. This reaction is considered to be minimized if its extent is sosmall as not substantially to affect the measurements of oxygen andcarbon dioxide concentration recorded. Three major factors are involvedhere, and each will be discussed in turn:

The size of the working electrode;

The rate of potential sweep;

Other characteristics of the potential sweep profile.

The working electrode is specified as having a surface area below 10000square microns (i.e. 100 μm)² ; above this figure it is difficult orimpossible to avoid increasingly significant interference due toreaction (2) above. The working electrode surface area is preferablybelow 1000 square microns, since above this value a rather high rate ofpotential sweep may be necessary in order to avoid interferingreactions.

The working electrode Surface area is preferably no more than 100 squaremicrons (i.e. 10 μm)² ; as shown below, good results can be obtainedunder these circumstances without any limitation on the rate ofpotential sweep. More preferably, the working electrode surface area isless than 10 square microns; as shown below, such electrodes can givemore accurate results with less correction required to compensate forinterfering reactions.

It is expected that working electrodes with surface area below 1 squaremicron will have added advantages. A sheet of insulating material maycarry an array of such electrodes, for example in pores extendingthrough the sheet. The total current will be the product of eachindividual cathode current and a number of cathodes, and redundancy willbe built in if some cathodes fail. Microelectrodes will reducegas/liquid difference effects, and will allow thin membranes to be used,thus decreasing the time response of the sensor. Microelectrodes willneed to be spaced apart, e.g. at least 5 and preferably at least 20cathode radii, from each other so as to avoid cross-talk effects betweenadjacent electrodes. There is in principle no lower limit on the size ofa microelectrode.

The rate of potential sweep is another important factor. If the rate istoo fast, it is found that the results obtained are not accurate orreproducible. With existing equipment, a preferred maximum rate of sweepis 50 V/s although higher rates may be possible with more sophisticatedequipment. As the rate of potential sweep is slowed, two problemseventually arise. The first is simply that the information (required todetermine gas concentrations) takes longer to obtain. In cases wherespeed of response is important, a rate of sweep of at least 0.01 V/s,e.g. 0.1 to 10 V/s, is likely to be preferred.

The other problem is that, in the case of a mixture of O₂ and CO₂ gases,if the rate of potential sweep is too slow, interference may arise fromthe unwanted reaction (2) above between carbon dioxide and superoxideion. Our work has shown that, provided the surface area of the (or each)working electrode is no more than 100 square microns, this problem doesnot arise in practice. Under these circumstances, the optimum rate ofpotential sweep is one which generates accurate information as quicklyas possible, and is likely to be in the range of 0.1 to 10 V/s. Withworking electrodes with larger surface area, other rates of sweep may beappropriate.

Other characteristics of the potential sweep profile are subject to manyvariations. In a simple case, for example, the potential of the workingelectrodes starts at -0.5 V, is raised to -2.5 V at a rate of 0.5 V/s,is reduced to -0.5 V at a rate of 0.5 V/s, and is immediately againraised to -2.5 V at the same rate and so on. The two end figures, of-0.5 V and -2.5 V, can be varied, provided only that the potential rangecovers the reduction potentials of the two gases concerned. The rate atwhich the potential is changed from -2.5 V to -0.5 V can be higher, e.g.infinite. The rates of sweep do not have to be linear, there may bepauses, at either end or intermediate the ends of the range.

Improved results may be obtained if the working electrode ispre-conditioned immediately before measurement is started. For example,the working electrode may be pre-conditioned at some potential withinthe sweep range, e.g. -1.0 V or -1.9 V for a short period, e.g. 1 to 60s, prior to measurement.

The device and method of this invention enable the concentrations of anumber of gases to be determined, using the same electrode and solvent,by sweeping the appropriate voltage range. They are suitable not onlyfor the determination of blood-gas concentrations, but for a variety ofother gases as well. It is envisaged that apparatus based on thisapproach could also be used for more general vapour analysis and as avolatile agent monitor. Applications would include the medical field,for example on anaesthetic machines, and in the food industry.

BRIEF DESCRIPTION OF THE FIGURES

Reference is directed to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a sensor according to the invention.

FIG. 2 is a schematic diagram of a bench reaction cell which includes aworking electrode (but not behind a membrane).

FIGS. 3 and 4 are graphs of current against voltage obtained using aparticular working electrode.

FIGS. 5, 6 and 7 are corresponding graphs of current against gasconcentration obtained using the same working electrode.

FIGS. 8 and 9 are graphs of current against voltage obtained using asecond working electrode.

FIGS. 10 and 11 are graphs of current against gas concentration obtainedusing the second working electrode.

FIGS. 12, 13 and 14 are further graphs of current against voltageobtained using a working electrode in the bench reaction cell.

FIG. 15 is a graph of current against gas concentration (CO₂) obtainedusing a working electrode in the bench reaction cell.

FIGS. 16 and 17 are further graphs of current against voltage obtainedusing an alternative working electrode in the bench reaction cell.

FIGS. 18 and 19 are graphs of current against gas concentration (N₂ Oand O₂, respectively) obtained using the alternative working electrodein the bench reaction cell.

FIGS. 20 to 23 are further graphs of current against voltage for variousanaesthetic gases obtained using the alternative working electrode inthe bench reaction cell.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

The sensor shown in FIG. 1 comprises a generally tubular PTFE body 10,one end of which is closed by means of a PTFE membrane 12 sealed to thebody by an "O" ring 14. A working electrode comprises a glass rod 16Containing an axially extending 10 μm diameter gold wire 18, the exposedend 20 thereof constituting the working electrode. The glass rod ispositioned in the PTFE body with the working electrode adjacent theinner surface of the PTFE membrane. A region 22 surrounding the glassrod and retained by the PTFE body 10 and membrane 12 is filled withdimethylsulphoxide. An Ag/AgCl reference electrode 24 or an Agquasi-reference electrode is also in contact with the DMSO. A DMSOoverspill hole 26 is provided in the PTFE body. Leads 28 and 30 connectthe working electrode and the reference electrode respectively to apower source (not shown) and an ammeter (not shown).

In use, the sensor is presented to a gas stream 32, for examplecontaining oxygen and carbon dioxide, portions of which pass through thePTFE membrane 12 and become dissolved in the DMSO solvent 22. Thepotential of the working electrode 20 is scanned at a suitable ratecyclically between, say, -0.2 V and -2.3V, and the current flowing atparticular potentials noted. For example, the current flowing at -0.8 Vmay be indicative of the oxygen concentration of the gas, while thecurrent flowing at -1.9 V may be indicative of the carbon dioxideconcentration in the gas. The optimum potential, or range of potentials,at which current is measured may depend to some extent on the particularcharacteristics of the apparatus. The total current flowing will bedependent, among other things, on the surface area of the workingelectrode. It will generally be necessary to compare the currentsgenerated by an unknown gas stream with the currents generated by gasstreams of known composition.

The reaction cell shown in FIG. 2 comprises a glass body 33 containing amixture of DMSO and TEAP as solvents 34. A working electrode comprises aglass rod 35 containing an axially extending gold wire 36, the exposedend 37 thereof constituting the working electrode. The glass rod ispositioned in the glass body with the working electrode submerged in theDMSO/TEAP solution. An Ag/AgCl reference electrode 38 or an Agquasi-reference electrode and a counter-electrode 39 are also in contactwith the solvent mixture. The three electrodes are connected by leads 40to a potentiostat (not shown).

In use, the gases/vapours under analysis are introduced into thereaction cell through an inlet pipe 41 and become dissolved in theDMSO/TEAP solvents. The cell volume is typically less than 1 ml so thatthe vapour/liquid equilibration time is reduced to a minimum. Workingelectrodes in which the gold wire had a diameter of either 2 μm or 10 μmwere available for use. The potential of the working electrode isscanned at a suitable rate cyclically and the current flowing atparticular potentials noted, as with the device of FIG. 1.

It will be appreciated that the reaction cell of FIG. 2 works on thesame principle as the sensor of FIG. 1, except that in the latter theworking electrode is behind a membrane. Thus, results obtained with thereaction cell would be expected to be reproduced when the samegas/vapour mixture is analysed using the sensor device according to thisinvention.

EXAMPLE 1

FIGS. 3 to 7 represent results obtained with the sensor shown in FIG. 1.The working electrode was a gold wire of 10 μm diameter. The membranewas of 12 μm PTFE. Measurements were made at room temperature, with thesensor inserted into a flowing gas stream of oxygen and carbon dioxidein nitrogen. Cyclic voltammograms were obtained at sweep rates of 0.5V/s, with a pre-conditioning signal of -1.0 V for 10 s prior to thevoltage sweep.

FIG. 3 is a voltammogram obtained at constant 10% v/v oxygen and 3, 6and 9% carbon dioxide, balance nitrogen. FIG. 4 is the same type ofvoltammogram at constant 30% oxygen and 3, 6, 9 and 12% carbon dioxide.In both graphs, an oxygen current can be read at -0.8 V, with noevidence of any carbon dioxide feedback effect on the oxygen signal. Acarbon dioxide current can be read at -1.9 V.

FIG. 5 is a graph of oxygen current against carbon dioxideconcentration, data having been taken from graphs such as FIGS. 3 and 4at -0.8 V. It is apparent that the oxygen current is not significantlyaffected by the carbon dioxide content of the gas.

FIG. 6 is a graph of carbon dioxide current (after subtraction of theoxygen current) against oxygen content for three different levels ofcarbon dioxide concentration. Again, it is apparent that the carbondioxide current is not significantly dependent upon oxygen concentrationof the gas stream. The same data is replotted in FIG. 7 in the form of agraph of carbon dioxide current against carbon dioxide concentration.Apart from experimental scatter at 3% carbon dioxide, good linearity isdemonstrated.

EXAMPLE 2

FIGS. 8 to 11 show results obtained in a rather different system. Theelectrode was constituted by the exposed end of a 2 μm diameter goldwire. The gas was dried and its temperature controlled at 37° C.Otherwise conditions were as for Example 1.

FIG. 8 is a graph of voltage against current for gas containing 10% v/voxygen and 3, 5.7 and 8.5% carbon dioxide. FIG. 9 is a correspondinggraph obtained using gas containing 20% oxygen.

FIG. 10 is a graph (obtained from data such as that shown in FIGS. 8 and9) of oxygen current against oxygen concentration, the currentmeasurements having been made at -0.85 V. Good linearity isdemonstrated.

FIG. 11 is a graph (obtained from data in FIG. 9) of carbon dioxidecurrent (with oxygen current subtracted) against carbon dioxideconcentration. The measurements were made at -1.7 V potential. Again,good linearity is demonstrated.

EXAMPLE 3

FIGS. 12 to 15 illustrate the measurement of three gases at the sametime and were obtained using the bench reaction cell shown in FIG. 2.

FIG. 12 is a graph of voltage against current for a gas mixturecontaining 10% v/v oxygen and 5, 10, 15 and 20% carbon dioxide, with thebalance being nitrous oxide. The oxygen signal is very low compared tothe carbon dioxide and nitrous oxide signals, but is more clearly seenin FIG. 13 (which also magnifies the carbon dioxide current). In FIG.14, both the voltage scale and the carbon dioxide current are magnifiedfurther. FIG. 15 is a graph of carbon dioxide current against carbondioxide concentration, and shows that the carbon dioxide signal islinear with concentration. Other results (not presented) demonstratethat both the oxygen and nitrous oxide current are also linear with therespective concentrations of those gases.

EXAMPLE 4

FIGS. 16 to 19 illustrate the measurement of oxygen and nitrous oxide inthe absence of carbon dioxide, and were obtained using the benchreaction cell shown in FIG. 2.

FIG. 16 is a graph of voltage against current for gas containing amixture varying from 0 to 100% v/v of oxygen and nitrous oxide. It willbe seen that the nitrous oxide currents are massive compared to theoxygen currents. In FIG. 17, the oxygen current is magnified.

FIGS. 18 and 19 are graphs (obtained from data shown in FIGS. 16 and 17)of current against nitrous oxide concentration and oxygen concentration,respectively. Good linearity is demonstrated.

EXAMPLE 5

FIGS. 20 to 23 illustrate the determination of concentrations of fourvolatile agents, namely the anaesthetic gases: enflurane, halothane,isoflurane and sevoflurane, and were obtained using the bench reactioncell shown in FIG. 2.

FIG. 20 is a graph of voltage against current for nitrogen gascontaining 0.6% v/v of isoflurane, pure N₂ gas being also shown forcomparison.

FIGS. 21 and 22 are graphs of voltage against current for 1) N₂ ; 2) 1%v/v of enflurane in nitrogen and 3) 1% v/v of enflurane in a 33% oxygen:67% nitrogen mixture. The oxygen reduction current is seen to be aminute fraction of the reduction current for enflurane.

FIG. 23 is a graph of voltage against current for each of enflurane,halothane, isoflurane and sevoflurane, all on the same voltage scale,giving a comparison of their respective reduction potentials:

1) ≈1% v/v sevoflurane (after 5 min) in 33% O₂ /67% N₂.

2) ≈1% v/v enflurane (after 5 min) in N₂.

3) ≈0.6% v/v halothane (after 6 min) in N₂.

4) ≈0.6% v/v isoflurane (after 3 min) in N₂.

These results demonstrate that all four of the anaesthetic gases underinvestigation have been successfully electrochemically reduced (on goldmicrocathodes in DMSO) using the device and method of the presentinvention. This is considered to be a surprising finding since previousteachings have indicated that only halothane can be reduced with anyease.

We claim:
 1. A method of determining the concentrations of at least onegas in a fluid, which method comprises the steps of:applying the fluidto one side of a membrane permeable to said at least one gas, the otherside of the membrane retaining a solvent for said at least one gas,using a working electrode having a surface area less than 10000 squaremicrons in contact with the solvent and applying a potential which isswept over a range effective to reduce said at least one gas in thesolvent, wherein the rate of sweep of potential is sufficient tominimize the interfering effect of any reaction between said at leastone gas and a reduction product of any other gas, measuring the currentgenerated at a predetermined potential as an indication of theconcentration of said at least one gas, and, where the concentrations oftwo or more gases are being determined, measuring the current generatedat each of one or more further predetermined potentials as an indicationof the respective concentrations of the one or more other gases present.2. A method as claimed in claim 1, wherein the working electrode surfacearea is less than 100 square microns.
 3. A method as claimed in claim 1,wherein said at least one gas comprises oxygen and carbon dioxide.
 4. Amethod as claimed in claim 1, wherein said at least one gas comprisesoxygen, carbon dioxide and nitrous oxide.
 5. A method as claimed inclaim 1, wherein said at least one gas is an anaesthetic gas.
 6. Amethod as claimed in claim 1, wherein said at least one gas comprises ananaesthetic gas together with at least one of oxygen, carbon dioxide andnitrous oxide.
 7. A method as claimed in claim 1, wherein the solvent isdimethylsulphoxide.
 8. A method as claimed in claim 1, wherein the rateof sweep of potential is from 0.1 to 10 V/s.
 9. A method as claimed inclaim 1, wherein the working electrode is pre-conditioned by being heldat a potential within the sweep range immediately prior to measurement.